Restore previous 'git describe' behaviour as discussed here:
http://lists.basho.com/pipermail/rebar_lists.basho.com/2012-September/001713.html

These functions will be necessary in rebar_eunit.erl, too.

It is now possible to call rebar as:

  rebar xref skip_app=Mod1,Mod2,...

This makes it easy to skip running xref on (e.g)
imported dependencies in your application.

The function rebar_utils:is_skipped_app/0 is added so
that other rebar commands may use it.

The function may fail with a badarg exception because the first regex
returns an iolist() which is allowed to be a improper list. In this case
'++' cannot append to the iolist. The correct way to append something to
an iolist() is

  [iolist(), "tail"]

because iolist's are allowed to be arbitrarily deep lists.

The introduction of setup_env as a global concept caused the rebar_port_compiler
implementation to start getting called a LOT. The expansion of environment variables
that happens in the port compiler was O(n^n), which means you could see upwards of
80k invocations of lists:foldl on a single app "./rebar clean". This commit reworks
the expansion to be O(n^2), and reduces the running time for the same operation by
60%+. On a large project like Riak, the end result is that a build went from 200
seconds to 73.